This investigation was designed to study the late effects of fast neutrons with a mean energy of 15 MeV on the brain, spinal cord, or lung of dogs irradiated with the same fractionation schedule currently being used for cancer management, compared with the late effects of megavoltage x-irradiation delivered in a clinical schedule. The late eefects on brain or spinal cord are being quantitatively evaluated by neurological exams, computerized axial tomography, and histopathology. Radiation changes in the lung are being monitored through radionuclide studies of perfusion, ventilation, aerosol deposition, and computerized axial tomography. Histopathological evaluation of the irradiated organs is underway. The onset, incidence, and degree of malignancy of neoplastic tissue is being documented and evaluated by radiography, computerized axial tomography and histological evaluation.